Polysaccharides are becoming increasingly important, as they are materials that can be obtained from renewable raw materials. One of the most frequently occurring polysaccharides is cellulose. Cotton fibers, which consist almost exclusively of cellulose, are an example of the significance of polysaccharides. However, also materials obtained from other cellulosic raw materials, e.g., cellulosic synthetic fibers, are continuing to gain in importance. In addition to the fibers, there also exist other cellulosic shaped bodies; an example thereof is cellophane sheets. At present, cellulosic sheets are produced almost exclusively according to the cellophane method and are stretched one-dimensionally.
The generic name “lyocell fibers” was assigned by BISFA (the International Bureau for the Standardization of Man-made Fibers) to cellulose fibers produced from solutions in an organic solvent without the formation of a derivative.
However, to date, only one method for the large-scale production of fibers of the lyocell type has gained acceptance, namely, the amine-oxide process. In this method, a tertiary amine oxide, preferably N-methylmorpholine-N-oxide (NMMO), is used as the solvent.
Tertiary amine oxides have long been known as alternative solvents for cellulose. From U.S. Pat. No. 2,179,181, it is known, for example, that tertiary amine oxides are capable of dissolving pulp without derivatization and that cellulosic shaped bodies, e.g., fibers, can be made from these solutions. U.S. Pat. No. 3,447,939 describes cyclic amine oxides as solvents for cellulose.
From numerous patent specifications and other publications, it has been known to those skilled in the art for quite some time how this method is carried out. EP 356 419 B1, for example, describes how the solution is prepared, and EP 584 318 B1 describes how such solutions of cellulose in water-containing tertiary amine oxides are spun into fibers. DE 10261496 A1 describes the production of flat cellulosic films from such solutions, while WO 00/23250 A1 describes the production of blown cellulosic films from comparable solutions.
Cellulosic sheets made according to the amine oxide process have clear product advantages over cellophane: they possess high strength in the longitudinal and transverse directions and are, depending on their manufacture, isotropic or anisotropic.
DE 10035798 A1 describes the production of cellulosic tubular films according to the amine oxide process. In its specification (column 5, line 13), it states 50-95% by weight, particularly 70-80% by weight, as possible NMMO concentration. In all examples of DE 10035798 A1—just like in all other publications on this topic—a cellulose solution using NMMO monohydrate (87% by weight of NMMO) as a solvent is invariably used in specific embodiments. When cellulose is used as a polysaccharide, other solution compositions, that is, those having higher contents of water, can simply not be processed. Typically, for slurrying the cellulose prior to dissolving it, an aqueous NMMO solution with a higher water content is employed, and during the dissolution process in the dissolving unit the excess water is evaporated by applying negative pressure. This evaporation requires a great deal of energy and investments for the vacuum generating system.
DE 10029044 A1 also describes the production of cellulosic shaped bodies, e.g., fibers and films, according to the amine oxide process. The spinning solution may for example be produced in a filmtruder or LIST Discotherm kneader under the simultaneous evaporation of water. Also in this case, the water content of the finished spinning solution prior to the extrusion is to be 11% by weight related to the total quantity of the spinning solution. Also in this case, a great deal of evaporation energy is needed.
Even recent scientific publications do not come to any other conclusion. Singha, International Journal of Materials Engineering 2012, 2(3): 10-16, describes for the dissolution of cellulose in aqueous NMMO in great detail by means of a dissolution diagram that with a water content of more than approx. 22% by weight in the overall system cellulose will not be dissolved.
The main cellulosic raw material used in the amine oxide process is pulp obtained from wood. The cellulose molecules existing in wood and also in other plant-based sources of cellulose such as cotton linters, straw, etc. form very long chains, i.e., they exhibit a high degree of polymerization. In order to obtain a cellulose spinning solution that is well suited for large-scale processing, it is necessary to specifically adjust the degree of polymerization of the cellulose molecules, which inevitably causes part of the polymer molecules to be shortened. This takes place in the usual pulp preparation methods and also in separate pretreatment steps such as bleaching, acid treatment, or irradiation by splitting the originally long cellulose molecules. In addition to the shorter chains having the desired degree of polymerization, this also creates significantly shorter fragments such as oligomers or even monomers which remain in solution after the precipitation of the spinning solution in the precipitation bath, do not contribute to the formation of the fibers, and thus are lost. The quantities of raw material lost in this way can be substantial and can affect the cost-effectiveness of the entire amine oxide process.
U.S. Pat. No. 7,000,000 describes fibers produced by spinning a solution of polysaccharides which substantially consist of repeating hexose units linked via α(1→3)-glycosidic bonds. These polysaccharides can be produced by bringing an aqueous solution of saccharose into contact with GtfJ glucosyltransferase, isolated from Streptococcus salivarius (Simpson et al. Microbiology, vol. 41, pp 1451-1460 (1995)). As used in this context, “substantially” means that within the polysaccharide chains there may exist occasional defective locations where other bond configurations may occur. For the purposes of the present invention, these polysaccharides are referred to as “α(1→3)-glucan”.
U.S. Pat. No. 7,000,000 first discloses possibilities for the enzymatic production of α(1→3)-glucan from monosaccharides. In this way, relatively short-chained polysaccharides can be produced without the loss of monomer units, as the polymer chains are built from the monomer units. Contrary to the production of short-chained cellulose molecules, the production of α(1→3)-glucan keeps getting less expensive the shorter the polymer chains are, as in that case the required residence time in the reactors will be short.
According to U.S. Pat. No. 7,000,000, the α(1→3)-glucan is to be derivatized, preferably acetylated. Preferably, the solvent is an organic acid, an organic halogen compound, a fluorinated alcohol, or a mixture of such components. These solvents are costly and complex to regenerate.
Therefore, attempts were made to use α(1→3)-glucans instead of cellulose for the production of polysaccharide fibers in an amine oxide process under commercially applied large-scale process conditions. Unfortunately, it was found that under these conditions α(1→3)-glucans could not be processed satisfactorily into fibers. In particular, the individual fibers would frequently stick together, making them unmarketable.
Object
In view of such prior art, the object was to provide polysaccharide films and a method for their production, which did not include the above mentioned disadvantages. In particular, the production method was to be more cost-effective to execute as compared to the production of cellulose films according to the amine oxide process.